1. Field of the Invention
The present invention relates to a light exposure mask for semiconductor devices, and more particularly to a light exposure mask capable of exhibiting a constant light contrast throughout all mask portions when a light exposure is carried out in accordance with a modified illumination method, thereby achieving an improvement in process yield and operation reliance.
2. Description of the Prior Art
The recent trend to fabricate highly integrated semiconductor devices has been greatly affected by the development of techniques of forming patterns having a micro dimension. Photoresist film patterns formed by a photolithography process are widely used as masks for carrying out an etch process or ion implantation process in the fabrication of semiconductor devices.
Generally, micro patterns for semiconductor devices are formed by uniformly coating a photoresist solution, which consists of a resin solved in a solvent of a certain amount along with a photoresist agent, over a semiconductor substrate to form a photoresist film, and then selectively irradiating light onto the photoresist film by use of a photoresist film pattern mask, thereby exposing, to the light, the portion of the photoresist film except for its portion occupied by the mask to form a photoresist film pattern. Thereafter, the light-exposed portion of the photoresist film is removed using an alkali development solution, thereby forming a photoresist film pattern. Using the photoresist film pattern, a conduction layer disposed beneath the photoresist film pattern is then etched, thereby forming a micro pattern.
With regard to such a micro pattern, the wiring width and space between neighboring wirings, namely, the line/space dimension can be adjusted by the photoresist pattern. However, this general technique is difficult to form a micro pattern having a critical dimension smaller than a certain dimension because of various limitations on the accuracy of the light exposure device and wavelength of light.
Conventional steppers, for example, using G-line, i-line and CrF excimer lasers respectively having wavelengths of 436, 365 and 248 nm as their light sources have resolutions only capable of forming patterns having line/space dimensions of about 0.7, 0.5 and 0.3 .mu.m, respectively.
In order to form micro patterns having a critical dimension smaller than the limit of the resolution of the above-mentioned steppers, various methods have been proposed which use a light exposure device with a light source of a short wavelength or with an increased accuracy, or use a phase shift mask as the light exposure mask.
A modified illumination method has also been proposed to achieve an improvement in light contrast by utilizing an interference between two light beams, namely, the 0th light beam and the +1'st or -1'st light beam both passing through a light transmission lens. In accordance with this method, the central portion of light incident on the light exposure mask is shielded in an annular or quadrapole shape.
Such a method is effective in providing an improvement in light contrast to line/space-repeated patterns.
FIGS. 1A to 1C are plan views of a light exposure mask employed to fabricate a semiconductor device in accordance with the modified illumination method, respectively taken at different positions.
FIG. 1A is a plan view of a portion of the light exposure mask corresponding to the memory region of the semiconductor device. Referring to FIG. 1A, repeated chromium patterns 2 having a certain uniform line/space dimension are formed on a transparent substrate 1.
On the other hand, FIGS. 1B and 1C are plan views respectively showing portions of the light exposure mask corresponding to peripheral circuit regions of the semiconductor device. These mask portions are provided with non-uniform patterns 3 or an independent pattern 4 including contact holes or inner wirings.
FIG. 2 is a graph illustrating light intensities respectively exhibited at different portions of the light exposure mask.
Referring to FIG. 2, it can be found that the repeated patterns disposed at the central portion 2A of the mask exhibit a higher light intensity than those disposed at the peripheral portion 2B of the mask. Thus, the repeated patterns 2 disposed at the peripheral mask portion 2B exhibit a reduced light contrast as compared to those disposed at the central mask portion 2A.
On the other hand, the non-uniform patterns 3 exhibit a reduced light contrast as compared to the repeated patterns 2. The independent pattern 4 has a lower light contrast than the non-uniform patterns 3. Thus, the light contrast varies depending on portions of the semiconductor device where the above-mentioned conventional light exposure mask (see FIG. 1) is used.
Where such a light exposure mask exhibiting different light contrasts for the same line width is used, setting of optimum energy with reference to the repeated patterns 2 disposed at the central mask pattern 2A results in the residue of the photoresist film material at spaces defined by the patterns of the peripheral mask portion 2B and non-uniform patterns 3. On the other hand, when the optimum condition is set with respect to the non-uniform patterns 3 exhibiting the smaller light contrast, there is a problem that the line width of the repeated patterns 2 is undesirably reduced. Since the photoresist film has a variation in thickness by the topology of the wafer, a bulk effect is generated which causes the processing steps to be instable.